Our model is the male rat subjected to chronic complete heart block at two different ages, 5 and 12 months, compared to age-matched control. We studied the response of contractile proteins to calcium by disrupting cell membranes of isolated muscle strips with the non ionic detergent Triton X-100 and then determining calcium dose-response curves for force development. Our results indicate that significantly more contractile force per unit of muscle area is developed by young, comared to older, animals with heart block or to control animals of either age. Thus an interaction of age and chronic volume overloading of the heart by complete heart block is demonstrated.